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Thermal cutout (compressor overheat) could be caused by high superheat. High pressurelockout means the head pressure was too high more than once and the board locked it out from trying to start again. If your illustrious "teacher" is not going into these details but is rather changing random parts, you are in need of better tutelage.

Learn to take superheat, subcool, temp diff, amp draw, cap readings, static pressure readings and you will go far in your troubleshooting ability. Until you understand what these readings mean, it will be difficult to troubleshoot HVAC.

High superheat on a txv system could mean the txv is not opening enough.

Do you think the fan was going on and off intermittently? I dont think it was the control board but wasn't sure. Its hard to determine the cause of a problem when the customer has had the system off for 24 hours and time is money you know!!

a good multi meter has a capacitor tester. Magnehelic gauge is a pressure gauge for the ductwork. It helps evaluate airflow.
fieldpiece makes a good multi meter with an amp clamp temperature capacitance all in 1.
Uei makes a cheap digital manometer(another name for magnehelic gauge)

I have installed many communicating systems and over the past year I have had to replace several high pressure cut out sensors. It seemed to only happen on the 4 ton 20i's. In one situation, it was going off on high head pressure in the heat mode due to a bad cond. txv. At the time the unit was brand new and it caused me and Trane alot of headache. It's been almost a year now with no problems. Read the manuals on how to ohm out the cut outs. At first it can be very frustrating but once you learn it, you will be a much better tech on these new systems. Good luck!

Not to be ignorant, however in school we were taught that the last thing a technician should do is put the gauges on. Rule out all other problems because you always lose a little refrigerant each time you put the gauges on and off. However in the situation the OP had I would have put the gauges on to see what the high pressure side was doing since it locked out on high pressure. However before I did that I would have checked the run capacitor on the condenser fan motor, checked to see if the fan turned easily, and I would have read the current on the condenser fan and checked it against the RLA to see if it was running harder than normal.

I am pretty new but I believe that this was probably caused by the fan not starting once and then the next time it started after you reset the board. I was on a service call once where the outdoor unit was not running. Turned out the compressor would start but the fan would not and it would cut out on high pressure. After checking the cap and windings I swapped the terminals and forced the fan backwards for a second and it loosened it up, after that it ran forward but took a bit to start. The current was about an amp higher than normal so we lowered the thermostat and let it run continuously (because we were afraid it wouldn't start again) until we could get a replacement motor a few hours later. So I guess my point is motors with worn bearings could start intermittently for several reasons.

Yes dirty dryer will effect super heat and subcooling. Some people mistake bad txv and it was plugged liquid dryer. Best thing is to keep reading and enjoy fixing things. You seem to care so in my book your ahead of alot.